This invention relates to dryers, and, more particularly, to a dryer for solvent based or water based inks and coatings which are applied to continuous webs by flexographic or gravure presses.
Present dryers for flexographic and gravure presses use hot air, sometimes with the assistance of infrared radiation, which impinges a moving freshly printed web. The temperature of the hot air is typically controlled so as not to exceed temperatures where the print, coating, or web may be compromised, including skinning of print or coating, boiling of print or coatings, or thermal yielding of film webs.
In general, traditional forced hot air drying systems used on flexographic and gravure printing and coating equipment have used slotted air impinging nozzle dryers. By impinging it is meant that the direction of the air stream flow has a predominant perpendicular velocity component relative to the local planar surface of the web being impinged upon by the air stream. The nozzle slot width on these systems has typically been in the range of 0.040 to 0.125 inch and with a nozzle slot length of the maximum web width plus or minus approximately 1 to 11/2 inch based on a particular application. The internal nozzle chamber pressures have typically been in the range of 5 to 15 inches water column (1 psi=27.76 inches water column) which produces the driving force to achieve impinging air flow velocities in the range of 5,000 to 12,000 feet per minute. The drying capacity of the system is dominated by the heat transfer characteristics in the locale of the impinging air stream. The heat transfer coefficient is strongly related to the impinging air stream velocity. Improving the performance of the traditional air impinging nozzle dryer technology is currently limited by technological, economical, and space limitations of the mechanics for which these systems are integrated.
Variations of the slotted nozzle arrangement include a distributed orifice array with orifice diameters of approximately 0.125 inch. Some dryer manufactures claim that such orifice arrays have improved evaporative drying performance. This particular type of configuration uses pressure supplies similar to the slotted nozzles described above.